Future Electronics – A new approach to designing flexible LED drivers

This article, by Peter Bensch, Technical Sales Manager, Future Electronics (Germany), describes an approach to the design of the power supply for a modular LED luminaire that combines low bill-of-materials cost, low EMI and considerable flexibility in production. Using a newly developed microcontroller that has features specially chosen for their applicability to LED power systems, this approach enables luminaire manufacturers to produce a broad range of products more cheaply than before, and thus profitably appeal to a more diverse range of customer requirements.

The inflexibility of standard power supplies

The electronics industry provides lighting equipment manufacturers with a broad choice of off-the-shelf, certified power-supply modules specially designed to drive LEDs. Integrating such a module is a quick and straightforward task. This is perfect for manufacturers that only want to produce LED luminaires with a known, fixed light output, since a standard LED power module typically has a fixed constant-current or constant-voltage output.

The market for lighting equipment, however, is both huge and diverse. Customers call for a dizzying variety of styles and sizes of luminaire with many different levels of light output. Meeting these diverse needs profitably is difficult if the manufacturer creates a wholly new power supply for every new product variant.

A more efficient approach is to create a modular power-system platform that can be easily and cheaply modified to supply a wide variety of end-products.

But now the market’s wide choice of standard, fixed-output power modules appears to be too inflexible, since different luminaires will require different power outputs. The point of a modular design, on the other hand, is that most components, including the power supply, should be common to all product variants.

There is another choice available to the system designer: a configurable power-supply module that offers a range of power outputs from one unit; the designer selects the required configuration through a simple end-of-line programming procedure. So a single configurable unit could support multiple variants of a luminaire design. Such programmable modules are available today from suppliers such as Philips Lighting, with its Xitanium range, and Roal.

A new modular architecture

Thanks, however, to the introduction of a new type of digital controller offering lighting-specific functions, a new modular power architecture can be implemented profitably by luminaire manufacturers. Since it operates like a microcontroller, it gives the designer the opportunity to easily integrate additional features that are valuable to a lighting application.

This new architecture uses the controller to switch between parallel strings of dimmable LEDs, as shown in Figure 1. The controller switches the LED strings in such a way as to minimise the input ripple current: this reduces the incidence of EMI while also keeping the Bill-of-Materials (BoM) and certification costs as low as possible.

Fig. 1: a single controller can reliably control the power supply to multiple LED strings

Different luminaire designs can be made by varying the number and length of the strings. An external AC-DC power module provides a constant voltage higher than the forward voltage of the longest LED string. Thus an almost infinite variety of luminaire power supplies can be realised with a single PCB design common to all.

As well as making system design simpler and accelerating time to market, this approach has another benefit: conventional high-intensity LED luminaires typically use a long string of LEDs in a serial configuration. The power supplies for such serial strings have very high voltages that require insulation for safety reasons. By contrast, multiple shorter strings of LEDs in parallel operate at a safe low voltage, and so require no protection against electric shock.

Preventing excessive EMI

The main issue facing the designer when implementing this architecture is to avoid the generation of harmful levels of EMI and ripple current arising from the many switching operations to which the board is subjected. It is normal to synchronise PWM drivers running at the same switching frequency to avoid interference and the potential for audible noise. In itself, this is quite easy to do. But it still leaves sensitive components on the board open to the risk of damage by high peak ripple currents.

Ripple current is generated at the edges of power switching cycles. In a system with several synchronised driver circuits, the ripple currents generated by the multiple simultaneous edges are combined, creating a large peak ripple current.

To avoid damage to other circuit components, the ripple current peak needs to be kept to a low value. A way to do this is to space out the falling edges, so that they do not occur simultaneously; the average ripple current stays the same, but the peak ripple current is lower. This interleaving can be achieved by phase-shifting the switching of the drivers.

In the past, the complexity of such a design might have been thought undesirable in an LED power supply. But the semiconductor industry has now stepped in to make the implementation of this kind of design much easier than before. The new MASTERLUX™ family of digital controllers from STMicroelectronics has been created to provide the functions required by today’s LED power designs, including phase-shifting.

In the STLUX385A, ST’s first MASTERLUX device, phase-shifting of switching operations is realised by an innovative digital technology called State Machine Event Driven (SMED). The SMED is a 96MHz state machine, implemented in hardware and triggered by internal or external events. It replaces and extends the functions that might otherwise be provided by analogue blocks in a traditional microcontroller.

Used as an LED driver controller, the SMED can close the regulation loop and automatically switch off the driver when it detects an overcurrent or a short circuit. Since SMED blocks are entirely implemented in hardware, they guarantee an event reaction time shorter than that provided by any standard interrupt-driven microcontroller processor core. This means that the required duty cycle for each LED channel will be executed on schedule, allowing for accurate and reliable regulation of light intensity.

The STLUX385A contains six SMEDs; each can be used to control one PWM LED driver. The driver itself is implemented outside the controller, for instance in a discrete buck regulator. STMicroelectronics provides a development tool for the SMED peripherals. It includes a powerful graphical SMED configurator which directly generates C code. The designer inputs the number of LEDs per channel, LED specifications and required average current through the LEDs, and the tool computes the required duty cycles and PWM configurations. The STLUX385A can also provide automatic compensation for the depreciation of the LEDs’ light output over time.

An example design

A useful way to see how the STLUX385A can be used in a multi-channel LED design is to study an example developed by STMicroelectronics. Its evaluation module STEVAL385LED4CH (described in detail in application note AN4291 at www.st.com) consists of four phase-shifted LED channels operating at an efficiency of up to 98%, all controlled by a single STLUX385A.

The evaluation board can be configured in a variety of ways, and thus provides a useful starting point for many luminaire manufacturers’ platform power designs. It will operate from any DC input between 12V and 48V. The power supply can drive and dim from three to ten LEDs per channel, with a maximum total power of 200W.

ST has chosen not to use the common buck-converter topology, as shown in Figure 2. Instead, this system uses a reversed buck topology to control a low-cost N-type MOSFET without boost capacitors. This reduces BoM cost, and also allows for the easy ground-referenced measurement of the switch current.

Fig. 2: common buck converter circuit (left side) and the reversed buck converter topology (right side) as used in the STLUX385A demonstration system

In the schematic for LED channel 0, as shown in Figure 3, L3, D4 and C11 form the reversed buck circuitry. D6 is a Schottky diode protecting the circuitry in case the LED string opens. The gate driver, U2, is used to drive the LED forward voltages: these are higher than the direct-drive capability of the STLUX385A, which is powered from a 3.3V supply rail, can support. The R28 and R29 shunt resistors gauge the switch current, supplemented by a spike filter formed by R23/C14. Q4 together with U5 capture the LED voltage, to support the protection features of the STLUX385A.

Fig. 3: the PWM signal from the STLUX385A controls a reversed buck converter at each LED channel

By configuring the PWM signal at the controller appropriately, each of the four LED channels’ power circuits can be phase-shifted to avoid generating high peak ripple currents, as shown in Figure 4.

Fig. 4: the SMED peripherals in the STLUX385A can be easily configured to phase-shift up to six LED channels’ switching operations

Additional useful features

The PWM control function described above is the core capability enabling the design of a modular power supply for LED luminaires. But a further advantage of using an integrated controller is that it provides additional features that are useful in an LED lighting system. These include:

Built-in support for control interfaces such as DALI, DMX-512, KNX (in tandem with an external KNX transceiver such as the NCN5120 from ON Semiconductor) and 1-10V control

Flexible ramp-up/ramp-down routines

Automatic adjustment in response to ambient light, when combined with a digital ambient light sensor such as the APDS-9301 from Avago Technologies, the BH1721FVC from ROHM or the TSL4531 from ams.

Error log

Conclusion

The STLUX385A provides all necessary control functionality to create a highly efficient, very flexible and powerful LED driver for luminaires. The proposed system solution keeps EMI within safe limits by using an interleaved switching scheme, and fulfils safety requirements by using a safe low voltage. At the same time, this solution is cheap to implement, because the switching scheme enables a single low-cost controller to control as many as six independent strings of LEDs.